CLEVELAND, Ohio--In the debate over the healthiest way to deliver a baby, the good old-fashioned, low-tech way recently gained more support.

A protein critical for brain development and function was triggered at a much higher level in babies born vaginally than in those born by cesarian section, in a study by Yale researchers.

The babies born by C-section also behaved differently from their peers born vaginally. They were more anxious, showed much less interest in exploring their surroundings and performed worse in a maze test. Yes, a maze test.

You might justifiably be scratching your head about now. That's because these babies were lab mice. There's still a significant amount of work to be done to see if the effect holds up in people, but the implications are important for several reasons.

First, C-sections accounted for 32.8 percent of deliveries in the U.S. in 2010, according to the most recent statistics from the Centers for Disease Control and Prevention. While these deliveries were down a tiny bit from the all-time high of 32.9 percent in 2009, they still account for a hefty number of births. Especially when you consider that only 5.5 percent of births were C-sections in 1970.

While the increase in C-sections ispartlybecause of older mothers having more-complicated deliveries and because of improved technology to detect problems during delivery, some of the increase is a matter of choice to make deliveries more convenient.

Second, several other recent findings have made a strong case for the biological importance of vaginal birth.

A May study by Harvard researchers found that human babies delivered by C-section were twice as likely to become obese. In a sample of 1,255 women recruited for the study, 284 gave birth by C-section. By age 3, 15.7 percent of these children were obese, compared with 7.5 percent of those delivered vaginally.

While it's unclear exactly what mechanisms are at work, the authors suggested that babies born vaginally develop different intestinal flora than those born by C-section. Their results were reported in The Archives of Disease in Childhood.

Premature babies born by C-section also had a 30 percent increased risk of respiratory distress, and no improvement in any other complications associated with prematurity compared with babies born vaginally, according to a recent study by researchers at Yale. This may be because babies born by C-section do not pass through the pelvis, which normally squeezes fluid from the lungs.

Last, Yale study author Tamas Horvath says the protein they looked at in their mice is a really important one that most likely acts in the same way in people.

It's called UCP2, for mitochondrial uncoupling protein 2, and it is important to the development of brain cells and the circuitry of the hippocampus, the part of the brain involved in learning and memory.

UCP2 is also involved in the cellular metabolism of fat, and babies, who survive mainly on a fat-rich diet of breast milk, have to metabolize a lot of fat. Horvath thinks UCP2 may therefore play some role in helping a baby transition to breast-feeding.

So does it work that way in people?

"It's a very primitive mechanism, so it's most likely going to be the same," he says. "I can put my neck on it being the same."

About 15 years ago, Horvath and his group were studying the protein, not at all interested in C-sections or natural birth, and stumbled on the vastly different levels in their lab animals delivered surgically or naturally.

"There was almost double the amount of protein in the natural-birth mice versus the C-section mice," he says.

If you eliminate the protein completely from the mice, by altering their DNA, they develop different behaviors, he says. Their experiments were preliminary, but it's enough to follow up on, he says.

"The implication is that there may be some consequence to a C-section birth down the line," he says.

Horvath is troubled by the trend of elective C-sections for convenience, which he says is growing quickly in many countries.

"When it's done for convenience, the question is whether this convenience is justified or you should consider whether natural birth has some major impact on human development as well," he says.

The team plans to expand their research to dogs, and eventually to people.

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